In modern handheld devices for cellular communication systems (e.g. 3GPP) there is a desire to support multiple frequency bands (e.g., 3GPP LTE bands 7, 1, 2, 3, 8, 5, and 13). Further in this regard, one particular area of interest has been for carrier aggregation (CA) radios, which provide the ability to support multiple receive carriers simultaneously. For example, such systems may be configured for transmitter and receiver frequency division duplexing (FDD), with transmitter and receiver(s) operating simultaneously at either one frequency separation or at varying frequency separation.
For small handheld devices, due to technology and size constraints, this kind of duplex operation has typically been achieved by switching between fixed frequency filters or duplex filters (e.g., dielectric coaxial resonator filters, SAW, BAW, FBAR) using semiconductor switches. In such a configuration, however, for each band of operation and for each combination of carrier aggregation pairs, a new set of hardware is needed (i.e., adding filters and switches when expanding band and carrier aggregation support).
In addition, for frequency division duplexing, an issue known as duplex self interference can arise from the high power of the transmitter challenging the linearity to which the receiver(s) can be set up with high gain to deal with low power reception levels. A spatial separation of receiver and transmitter antennas was the first way to deal with this issue.
Furthermore, for “over antenna duplex,” where transmitter and receiver have separate antennas such that some duplex isolation is created between the antennas, a filter is commonly provided in the receive path and is configured to primarily reject the transmit frequency to avoid overdrive conditions and minimize intermodulation products in the receiver. Similarly, a filter is provided in the transmit branch and is configured to primarily reject the transmitter noise at receive frequency.
For carrier aggregation FDD application (e.g., LTE Advanced), a further issue arises with the simultaneous reception of multiple bands while filtering away the transmit carrier. With prior art fixed frequency filter technology, bandpass filters have been used in each receive path. Because these filters will reflect signals out of the pass band, however, precautions have to be made so that the reflection or impedance of one filter passing a first signal band will not disturb the in-band signal match of a 2nd (or Nth) filter branch passing a 2nd (or Nth) signal band (and similarly in the opposite direction). For closely spaced CA bands and with prior art fixed frequency filter (e.g., acoustic filters) solution, the OEM cellular terminal manufacturer will have to select specific carrier aggregation pairs of bands that should be simultaneously operable and then integrate one or more diplex filter banks into the terminal. Pairs of bands that have not been configured will not be CA compatible.
Accordingly, it would be desirable for these kinds of systems to be able to support multiple frequency bands while addressing the above constraints of multiple hardware sets, duplex self interference, and the interaction of multiple receive pass bands.